Naphthenic acid esters as dispensing agents for pigment additives and products incorporating same

ABSTRACT

The present invention provides for novel pigment-dispersant compositions, incorporating various pigments, such as for example carbon blacks, comprising dispersants which are esters of naphthenic acid, and higher and lower analogs thereof. The invention also provides for, processes for the preparation thereof and coating and rubber compositions comprising same. The dispersing agents employed the present invention have been found to be particularly useful as dispersing agents for conductive carbon black and other pigments.

This is a division of application Ser. No. 08/014,213 filed Feb. 5, 1993now U.S. Pat. No. 5,383,965.

BACKGROUND OF INVENTION

This invention relates to the use of esters of naphthenic acids and thehigher or lower molecular weight analogs thereof, as dispersing agentsfor pigment additives such as carbon blacks, and encompassespigment-dispersant compositions, processes of preparation thereof andcoating and rubber compositions comprising a pigment thus dispersed. Thedispersants of the present invention permit the preparation of variouspigment dispersions which may be employed without adverse effects on thefinished coatings or rubbers into which such dispersions may beincorporated.

One of the most difficult pigments to disperse and stabilize in awaterborne coating formulations or rubber compositions is carbon black,because of its enormous surface area and high oil absorption.Additionally, especially where a conductive carbon black pigment is tobe utilized, it is important that the reduction of the electrostaticproperties be minimized, and that the other physical characteristics ofthe final coating are not deleteriously affected by the particularpigment dispersant employed.

Naphthenic acid has been employed as a dispersing agent in printing inkswhere it has been shown to be a good dispersant for carbon black, andwhere it can improve fluidity and decrease clogging and gummingtendencies. (Lowar, Spec. Chem., April, 1989) Also, reference has beenmade in the art to the use of metal soaps of this acid as usable in themanufacture of pigments for paint, glazes, etc. (Kirk-Othmer'sEncyclopedia of Chemical Technology, Volume 9, p. 245). However, theapplicants know of no prior art reference directed to the use of estersof naphthenic acids as dispersants.

The products commonly employed in the prior art as carbon blackdispersants in coatings are salts of an acrylic acid copolymer,acetylenic diol surfactants, or polyalcohol ethers which fit intovarious classes of wetting and dispersing agents, (Calbo, Handbook ofCoatings Additives, Dekker pg. 516). Such additives could be called onto function as more than a dispersant and can also act in one or more ofthe following ways:

a) to prevent flocculation

b) to prevent hard settling

c) to improve jetness/color/gloss

d) to control viscosity, and/or

e) to improve wetting of the base resin.

Various considerations are important in determining the usefulness ofany additive as a dispersing agent for use with a carbon black or withother pigments, depending upon the product into which such a dispersionis to be incorporated.

When used throughout this application the terms pigment(s) or pigmentdispersion(s) are intended to encompass various materials which may beintended to impart either color and/or serve some other function, suchas for example the use of carbon black in rubber where, in addition toadding color, such also acts as a reinforcing agent.

One of the most important considerations in determining whether aparticular dispersant will be useful for use with a given pigment orpigments when such a pigment is to be used in a paint or coatingcomposition is whether such a dispersant/pigment combination will orwill not impart a conductive nature or characteristic to the dried paintfilm or coating into which it has been added.

The automotive industry is replacing and will continue to replaceexterior metal body panels on vehicles with plastic and composite bodypanels. Some reasons for this change are weight reduction, flexibilityof design, and lower tooling costs. The replacement of metal body panelsby plastics and composites is not without difficulties.

One problem of note is the electrostatic spray painting of plastics.Electrostatic spray painting is the preferred manner of applyingautomotive topcoats. Spray painting normally gives the best appearanceto the vehicle and the electrostatic technique assures the mosteconomical use of the material. The problem arises because plastics donot paint well electrostatically unless a conductive primer is used.

Amongst the most important considerations for determining the utility ofany dispersant to be used in conjunction with conductive carbon blacksare the following:

the inherent rheological stability of the dispersion, both alone andwhen added to a formulated paint;

resistance to flocculation of the carbon black/dispersant mixture and inthe final paint or coating; and

ability to achieve low viscosity at high pigment loadings.

The various prior art references of which the applicants are aware whichrelate to dispersing agents for pigment additives, such as carbonblacks, suffer from a number of shortcomings.

The most significant shortcomings of the carbon black dispersants of theprior art, including those used for conductive carbon blacks, are:

high levels of dispersant may be required which tends to detrimentallyaffect the physical properties of formulated paints, such as adverselyaffecting the resultant humidity resistance, yellowing upon exposure toUV light, loss of cure in melamine cross-linked systems, and otherundesirable effects;

inability to prevent reflocculation of carbon black, resulting in theloss of electrical conductivity in dried paint films; and

incompatibility of the dispersant with the particular resin systemselected for use in the final paint formulation.

The shortcomings of the prior art dispersing agents noted above may beovercome by employing certain naphthenic acid esters in accordance withthe present invention.

It is therefore an object of the present invention to provide for novelcarbon black and other pigment dispersions.

It is also an object of the present invention to provide a process forthe preparation of pigment dispersions comprising the dispersants of thepresent invention for use in various coatings and rubbers.

It is a further object of the present invention to provide for improvedcoatings and rubbers incorporating the dispersants and dispersionstaught herein.

It is another object of the present invention to provide an improvedprocess for the production of various conductive coatings comprising theuse of the dispersants an dispersions of the present invention.

Lastly, it is an object of the present invention to provide an improvedprocess for the production of rubber compositions comprising the use ofthe dispersants and dispersions of the present invention.

These and other objects of the invention will be reflected in the courseof the following for more detailed discussion.

SUMMARY OF THE INVENTION

The present invention relates to novel dispersing agents for pigmentadditives, such as for example carbon blacks, comprising esters ofnaphthenic acid, and higher and lower analogs thereof. The inventionalso encompasses pigment-dispersant compositions and processes for thepreparation thereof, as well as coating and rubber compositionscomprising same.

It is anticipated that the dispersants of the present invention will bemost effective when used to disperse various organic pigments.

The dispersing agents of the present invention have been found to beparticularly useful as dispersing agents for carbon black and othersimilar organic pigments. Naphthenic acid ester dispersants exhibit thefollowing advantages, particularly when used with conductive carbonblack pigments:

excellent flocculation resistance of the dispersed black pigment;

increased grinding efficiency;

inherant rheological stability, that is low viscosity at high pigmentloading;

excellent dry film conductivity; and

the humidity resistance of the formulated paint or film is notmaterially affected by use of claimed dispersants.

These and other features and advantages of the present invention will bemore readily appreciated from the following more detailed description ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

The dispersants of present invention which have been found to beparticularly useful as dispersing agents for carbon blacks and otherpigments are certain esters of naphthenic acids and the higher and loweranalogs thereof.

These compounds generally have the chemical formula: ##STR1## where R'=anaphthene group containing one or more alkylated cyclopentane and/orcyclohexane ring(s), and

where

R=H or CH₃,

where

n=1-6, and

X=H or ##STR2##

Generally, --R' can be represented by the formula: ##STR3## where m isgreater than 1, preferably greater than 4 and less than 10, and

where the number of fused rings, n, may range from 1 to 5 and a smallfraction of these rings may be cyclohexyl. R' consists of at least C₁-C₃ aliphatic groups.

More particularly, R' may be a naphthene group containing one or morering structures selected from the group comprising methylcyclopentane,1,2-dimethylcyclopentane and methylcyclohexane.

Generally speaking, it is contemplated that a variety of esters ofvarious naphthenic acids will find utility as dispersants in accordancewith the teachings of the present invention depending upon thecompatibility of the particular naphthenic acid ester or esters and thepigment/coating or pigment/rubber system in which such is to beincorporated.

Naphthenic acids are described in Kirk-Othmer's Encyclopedia of ChemicalTechnology, Third Edition, Volume 15, pp. 749-753, especially pp.749-750, John Wiley & Sons 1981, the disclosure of which is incorporatedherein by reference.

Naphthenic acids are well known to the art and are availablecommercially, primarily as the result of various extractive procedurescarried out in refining naturally occurring petroleums and petroleumfractions, particularly gas-oil distillates boiling in the range of400°-700° F. Naphthenic acids are found particularly in petroleumsoriginating in the western part of the United States, especially inCalifornia; in the northern part of South America, especially Venezuela,Columbia and Aruba; in the southern part of Europe, especially Rumaniaand to a lesser extent, in crude oils originating in Louisiana, alongthe Texas gulf coast, in Peru, Mexico, Poland, the Soviet Union andGermany.

The chemical nature of these naphthenic acids is more or lessill-defined, but they all have in common the characteristic ofpossessing alicyclic nuclei. These may be five or six membered carbonatom ring systems, various condensed ring systems, and other relatedmaterials as disclosed in Fieser, "Organic Chemistry," 3rd ed. 1956, pp.97-98.

Various acid esters may be formed from the aforementioned class ofnaphthenic acids, by subjecting such acids to a well knownesterification reaction which will be familiar to one skilled in thisart, and requires no further elucidation here.

The specific acid esters formed will be a function of the particularnaphthenic acid or mixture of naphthenic acids utilized and theesterification reactant employed.

Such reactants are typically alcohols having 1-12 carbon atoms, and 1 to4 OH groups.

Amongst the preferred alcohols are diols having a mixed ether backbone,such as for example, 1,6-hexane diol and the like; triols, such asaliphatic triols having up to 10 carbon atoms, for example,trimethylolpropane, and monofunctional alcohols such as for example,methanol, propanol, butanol, and the like. Other preferred alcohols are:2-ethyl-1,3-hexanediol tetraethylene glycol, dipropylene glycol,tripropylene glycol, propylene glycol, and 1,5-pentanediol.

The esters employed, in the present invention are preferably "half-acidesters", such as TGN ester, wherein one end of the molecule acts as ananchor to be compatible with the resin, while the other end of themolecule acts as a surfactant to wet out the pigment.

Preferred naphthenic acid esters are prepared from mixed isomers ofcyclic aliphatic carboxylic acids, wherein the rings are mixed C₅ /C₆rings, having short, that is to say C₁ -C₃, aliphatic side chains eachof which will have a terminal carboxylic acid group, and isomersthereof.

Of particular interest is the ester of naphthenic acid, triethyleneglycol naphthenate (TGN) which has the following structure: ##STR4##wherein R'=the naphthene group containing an alkylated cyclopentaneand/or cyclohexane ring(s).

The general physical properties of TGN ester are the following:

    ______________________________________                                        Appearance           Amber Liquid                                             Color                Gardner 10, Max.                                         Specific Gravity @ 24° C.                                                                   1.00                                                     Viscosity @ 100° F.                                                                         42 cs.                                                   Flash Pt.            395° C.                                           Pour Pt.             -21° C.                                           Acid Number mg KOH/g 10, Max.                                                 Saponification Number                                                                              135                                                      Hydrophile-Lipophile 7.2                                                      Balance (HLB) Value                                                           ______________________________________                                    

Also, TGN ester is soluble in various organic solvents, included mineralspirits, toluene, ethyl acetate, isopropanol and methanol, that is,solvents which span the solubility parameter range of 7.7 to 14.5 cal1/2 cm-3/2, and are suitable for coatings formulations.

This material is commercially available from MERICHEM under thetradename Napester T™ and from AKZO Chemicals, Inc. under the tradenameVictaperse™.

The naphthenic acid ester dispersants of the present invention have beenfound to have particular utility as dispersants for conductive carbonblack pigments for use in conductive prime films and coatings, where itis known that optimum conductivity depends upon, among other parameters,good dispersion.

Over-dispersion can result in a diminished conductivity by shearing anddestroying chains through which electrons can travel.

A problem that is frequently encountered in the formulation of aconductive primer is the loss of dry-film conductivity as the film orcoating ages. This defect is attributed to postwetting of the carbonblack pigment by the vehicle or additives, thereby insulating theparticles from each other. Insulation, and hence loss of dry-filmconductivity, can also be encountered through the dispersion process ifthe dispersant (surfactant in aqueous systems) or vehicle chosen for themillbase is "too efficient" at wetting the carbon black aggregates.

A loss in dry-film conductivity can also be attributed to reflocculationof the carbon black aggregates in the wet paint due to incomplete or"unstable" dispersion of the carbon black aggregates,

It has been found that naphthenic acid esters having ahydrophile-lipophile balance (HLB) of 6 to 10 provide a stable, humidityinsensitive coatings that maintain dry film conductivity afteraccelerated aging conditions. Preferred naphthenic acid esters willgenerally have HLB's of from about 7-9. The most preferred range will befrom about 7-8.

Generally, the use of the dispersants of the present invention willallow one to achieve a higher effective incorporation of pigment perunit volume of coating without over mixing and therefore allow one topreserve the desirable characteristics of the pigment being utilized.

When employed as a dispersant for carbon black for incorporation in aconductive coating compositions the amount of naphthenic acid esters,such as triethylene glycol naphthenate, to be utilized to effectivelydisperse the pigment, while maximizing the conductivity of the coating,will vary from greater than 0 to 5 weight percent based upon the weightof carbon black. More preferably from about 0.9 to 4.8 weight percentbased upon the weight of carbon black utilized, will be employed.

In effect, the preferred dispersions of the present invention wouldallow one to achieve a greater effective dispersion of conductive carbonblack per unit of dispersant than would other available prior artdispersants.

Thus, a given quantity of dispersant will tend to disperse an optimumamount of conductive carbon black pigment without effecting thebreakdown of the structure of the carbon black, which is a function ofthe amount of mixing required to disperse the pigment, and thereforewill generally yield a higher conductivity and provide optimum dry filmconductivity.

Generally, it is expected that one will achieve a higher level ofpigment incorporation into elastomeric compositions, such as rubbercompositions, than would otherwise be possible using prior artdispersants.

One generally incorporates pigments in rubber compositions asreinforcing agents or to impart conductivity.

The advantages to be achieved via the use of the dispersants of thepresent invention versus known prior art dispersants for conductivecarbon black which is incorporated into various elastomeric materials,such as rubbers, will be commensurate with those which have been shownfor conductive carbon black pigments in various coating compositions.

When employed as a dispersant for the incorporation of organic pigmentssuch as carbon black into various elastomeric compositions, it has beenfound that optimum results can be obtained using from about greater than0 to about 10 weight percent of dispersant based upon the total weightof pigment to be dispersed in the elastomeric composition.

Preferably from about 1-7 weight percent dispersant based upon the totalweight of pigment will be employed and more preferably from about 2-3weight percent will be used.

The foregoing general discussion of the present inventions will befurther illustrated by the following specific examples.

EXAMPLES Examples 1-4

Into a quart plastic screw cap bottle filled with 355 pares of 6 mmglass beads was placed the following ingredients in order (parts referto gram weights):

    ______________________________________                                                       EXAMPLE NO.                                                                     1                                                                             Control  2       3     4                                     COMPONENT        PARTS                                                        ______________________________________                                        Acetylene Black  8.0      8.0     8.0   8.0                                   Microtalc        7.5      7.5     7.5   7.5                                   Acrylic Latex (50% NV)                                                                         53.7     53.7    53.7  53.7                                  N-Methylpyrrolidone                                                                            6.8      6.8     6.8   6.8                                   Butyl Carbitol   6.8      6.8     6.8   6.8                                   Deionized water  16.1     16.1    16.1  16.1                                  Triethylamine    1.32     1.32    1.32  1.32                                  Aqueous Ammonia (29%)                                                                          1.0      1.0     1.0   1.0                                   triethylene glycol                                                                             0        0.08    0.20  0.40                                  naphthenate                                                                   Imino methylated melamine                                                                      54.8     54.8    54.8  54.8                                  Polyurethane Dispersion                                                                        84.5     84.5    84.5  84.5                                  resin (35% NV)                                                                ______________________________________                                    

The above components were placed on a shaker mill and allowed to agitatefor the following times at which Hegman grind readings were taken:

    ______________________________________                                        TIME OF GRIND        HEGMAN GRIND READING                                     ______________________________________                                        20 minutes   2       3.2      3.5     4.25                                    30 minutes   3       4        4       5                                       40 minutes   4       4.75     4.75    5.5                                     50 minutes   4.5     5        5       --                                      ______________________________________                                         *the grind time to reach 5.5 Hegman was 35 minutes in this example.      

The dispersions from above were then let down with the followingingredients:

    ______________________________________                                        COMPONENT        PARTS                                                        ______________________________________                                        Polyurethane dispersion                                                                        84.5   84.5     84.5 84.5                                    resin (35% NV)                                                                Deionized water  5.0    5.0      5.0  5.0                                     ______________________________________                                    

The physical constants of the above formulated paints are shown in thefollowing Table 1. Constants were measured on paints that were appliedto Budd DSM950 SMC (Sheet molded compound) by conventional air atomizedsiphon gun spray application. Bake on the coatings was 280 degreesFahrenheit for 30 minutes. A second coat of the primer was then appliedover one half of the ambient cooled panel by the same application methodand baked according to the same oven schedule. A conventional solventborne high solids basecoat/clearcoat was applied to the bottom half ofthe panel (leaving the upper half of the panel exposed with the primerin one and two coats) in the same application method (at a basecoatthickness of 0.8 dry mils and wet-on-wet application of a clearcoat at1.8 dry mils) and baked 30 minutes at 250 degrees Fahrenheit.

                                      TABLE 1                                     __________________________________________________________________________    PHYSICAL PROPERTIES OF CURED PAINT FILMS                                                      PAINT OF EXAMPLE                                                              1     2     3     4                                           __________________________________________________________________________    Dry film primer                                                               thickness (mils):                                                             first coat      1.0   0.9   0.9   1.0                                         second coat     0.9   1.0   1.0   1.0                                         Dry film primer conductivity                                                                   165+  165+  165+  165+                                       Ransberg units                                                                Primer gloss (60 degree meter)                                                                 38    38    38    38                                         Initial crosshatch                                                            adhesion (% retained):                                                        (primer to substrate)                                                                         100   100   100   100                                         (primer to primer)                                                                            100   100   100   100                                         (topcoat to primer)                                                                           100   100   100   100                                         Crosshatch adhesion                                                           after exposure to 100%                                                        relative humidity for                                                         48 hours:                                                                     (primer to substrate)                                                                         100   100   100   100                                         (primer to primer)                                                                            100   100   100   100                                         (topcoal to primer)                                                                           100   100   100   100                                         __________________________________________________________________________

The net effect of the triethylene glycol naphthenate dispersant was tolower the amount of time required to attain a suitable grind of theconductive carbon black pigment without deleterious effects on thephysical properties of the dry paint film.

It is also noted that triethylene glycol naphthenate is considered agood dispersant in that it wets the pigment without nullification of theconductivity. It has been found that dispersants which in the pastallowed one to shorten the effective grind time, also destroyed theconductive nature of the pigment.

Examples 5-7 Comparative Examples

Into a quart plastic screw cap bottle filled with 355 parts of 6 mmglass beads was placed in the following ingredients in order (partsrefer to gram weights):

    ______________________________________                                                        EXAMPLE                                                                       5       6      7                                              COMPONENT         PARTS                                                       ______________________________________                                        Acetylene Black   8.0       8.0    8.0                                        Microtalc         7.5       7.5    7.5                                        Acrylic Latex (50% NV)                                                                          53.7      53.7   53.7                                       N-Methylpyrrolidone                                                                             6.8       6.8    6.8                                        Butyl Carbitol    6.8       6.8    6.8                                        Deionized water   16.1      16.1   16.1                                       Triethylamine     1.32      1.32   1.32                                       Aqueous Ammonia (29%)                                                                           1.0       1.0    1.0                                        Surfynol 104      0.4       0      0                                          Surfynol GA       0         0.4    0                                          Surfynol TG       0         0      0.4                                        Imino methylated melamine                                                                       54.8      54.8   54.8                                       Polyurethane Dispersion                                                                         84.5      84.5   84.5                                       resin (35% NV)                                                                ______________________________________                                    

The above components were placed on a shaker mill and allowed to agitatefor the following times at which hegman grind readings were taken:

    ______________________________________                                        TIME OF GRIND        HEGMAN GRIND READING                                     ______________________________________                                        20 minutes   2       3           3.5                                          30 minutes   3       4           4.25                                         40 minutes   4       4           4.5                                          50 minutes   4.5     5           5                                            ______________________________________                                    

The dispersions from above were then let down with the followingingredients:

    ______________________________________                                        COMPONENT       PARTS                                                         ______________________________________                                        Polyurethane dispersion                                                                       84.5       84.5   84.5                                        resin (35% NV)                                                                Deionized water 5.0        5.0    5.0                                         ______________________________________                                    

The physical constants of the above formulated paints are shown in thefollowing Table 2, Constants were measured on paints that were appliedto Budd DSM950 SMC (Sheet molded compound) by conventional air atomizedsiphon gun spray application, Bake on the coatings was 280 degreesFahrenheit for 30 minutes, A second coat of the primer was then appliedover one half of the ambient cooled panel by the same application methodand baked according to the same oven schedule. A conventional solventborne high solids basecoat/clearcoat was applied to the bottom half ofthe panel (leaving the upper half of the panel exposed with the primerin one and two coats) in the same application method (at a basecoatthickness of 0.8 dry mils and wet-on-wet application of a clearcoat at1.8 dry mils) and baked 30 minutes at 250 degrees Fahrenheit.

                  TABLE 2                                                         ______________________________________                                        PHYSICAL PROPERTIES OF CURED PAINT FILMS                                                     PAINT OF EXAMPLE                                                              5       6       7                                              ______________________________________                                        Dry film primer                                                               thickness (mils):                                                             first coat       1.0       0.8     0.9                                        second coat      0.9       1.0     1.0                                        Dry film primer conductivity                                                                   140       155     165                                        Ransberg units                                                                Primer gloss (60 degree meter)                                                                  38        38      38                                        Initial crosshatch                                                            adhesion (% retained):                                                        (primer to substrate)                                                                          100       100     100                                        (primer to primer)                                                                             100       100     100                                        (topcoat to primer)                                                                            100       100     100                                        Crosshatch adhesion                                                           after exposure to 100%                                                        relative humidity for                                                         48 hours:                                                                     (primer to substrate)                                                                           0         0       0                                         (primer to primer)                                                                              0         0       0                                         (topcoat to primer)                                                                            100       100     100                                        ______________________________________                                    

The net effect of the Surfynol dispersants was not as good as thetriethylene glycol naphthenate in either lowering the grind times neededto achieve the desired particle size or in providing a humidityinsensitive coating.

The dispersants also seemed to be detrimental to final dry filmconductivity versus the triethylene glycol naphthenate. This is probablydue to flocculation of the pigment in the wet paint as indicated by thepoor grind development over time versus the triethylene glycolnaphthenate.

While a limited number of preferred embodiments of the present inventionhave been described and tested above, one skilled in the art will,nevertheless, recognize numerous substitutions, modifications andalterations which can be made without departing from the spirit andscope of the invention as limited by the following claims.

We claim:
 1. An improved rubber composition which comprises rubber and apigment dispersion comprising a normally solid pigment component and anester of naphthenic acid, having the general formula: ##STR5## whereR'=a naphthene group containing one or more alkylated cyclopentaneand/or cyclohexane ring(s) , andwhere R=H or CH₃, where n=1-6, and X=Hor ##STR6##
 2. The rubber composition of claim 1 wherein in said pigmentdispersion, R' is a naphthene group containing one or more ringstructures selected from the group consisting of methylcyclopentane,1,2- dimethylcyclopentane and methylcyclohexane.
 3. The rubbercomposition of claim 1 wherein in said pigment dispersion, R' is anaphthene group having the general formula: ##STR7## where m is greaterthan 1, and where the number of fused rings, n, may range from 1 to 5and a small fraction of these rings may be cyclohexyl, and where R"consists of a C₁ -C₃ aliphatic group.
 4. The rubber composition of claim1 wherein in said pigment dispersion, the dispersant is an ester ofnaphthenic acid having the chemical formula: ##STR8## wherein R'=thenaphthene group containing one or more alkylated cyclopentane and/orcyclohexane ring(s).
 5. The rubber composition of claim 1 wherein saidpigment dispersion comprises a carbon black as the pigment andtriethylene glycol naphthenate as a dispersant.
 6. The rubbercomposition of claim 1 wherein said pigment dispersion comprises aconductive carbon black as a pigment and triethylene glycol naphthenateas a dispersant.
 7. A process for preparing an improved rubbercomposition comprising adding to a base rubber composition a pigmentdispersion comprising a normally solid pigment component and an ester ofnaphthenic acid, having the general formula: ##STR9## where R'=anaphthene group containing one or more alkylated cyclopentane and/orcyclohexane ring(s), andwhere R=H or CH₃, where n=1-6, and X=H or##STR10##
 8. The process of claim 7 wherein said dispersion comprises aconductive carbon black as a pigment and triethylene glycol naphthenateas a dispersant.
 9. A process for preparing an improved rubbercomposition comprising adding to a base rubber composition an amount ofa pigment dispersion effective to achieve optimum incorporation of thepigment without effecting the desirable pigment characteristics, whereinsaid pigment dispersion comprises a normally solid pigment component andan ester of naphthenic acid, having the general formula: ##STR11## whereR'=a naphthene group containing one or more alkylated cyclopentaneand/or cyclohexane ring(s), andwhere R=H or CH₃, where n=1-6, and X=H or##STR12##